1. Field of the invention
The present invention relates to a vehicle control device mounted to a vehicle for controlling a vehicle acceleration so that an actual acceleration (or a current acceleration) of the vehicle becomes equal to a target acceleration requested in response to a running state of the vehicle.
2. Description of the Related Art
There have been known various types of driver support systems for cruise control and adaptive cruise control. The cruise control keeps a vehicle speed at a set speed (or a target speed). The adaptive cruise control keeps a distance between a driver's vehicle and a front running vehicle at a predetermined vehicle distance (or a target vehicle distance) that is set in advance and changeable. Such a type of the driver support system is comprised of a power train control device and a hydraulic (or oil pressure) brake control device.
The power train control device generates a power train control value and controls a power train mechanism comprised of an internal combustion engine and a transmission to generate a drive torque or a brake torque in proportion to the magnitude indicated by the power train control value.
The hydraulic brake control device generates a brake control value and controls a hydraulic (or oil pressure) brake mechanism to generate a brake torque in proportion to the magnitude designated by the brake control value.
The driver support system further comprises a vehicle control device for executing following control processes (a) and (b):
(a) a process of calculating a target vehicle acceleration, that is required to obtain a target vehicle speed or to keep a target vehicle distance, in response to a running state of the vehicle (hereinafter, referred to as the “target acceleration calculation process”); and
(b) a process of calculating a power train control value and a brake control value so that the target vehicle acceleration calculated by the target acceleration calculation process (a) becomes equal to an actual acceleration applied to the vehicle, and of outputting the power train control value and the brake control value to the power train control device and the hydraulic brake control device.
Japanese patent documents, for example, Japanese unexamined patent laid open publication “Kohyo” No. JP 2006-506270, and Japanese patent laid open publication No. JP H07-81463 have disclosed such a vehicle control device.
When the target acceleration is a positive acceleration to increase the driver's vehicle speed, the vehicle control device of such a type calculates a power train control value so that the actual acceleration of the driver's vehicle reaches the target acceleration, and then outputs the calculated power train control value to the power train control device in order to generate a drive torque by the power train mechanism.
On the other hand, when the target acceleration is a negative acceleration (hereinafter, referred to as a “target deceleration”) to decrease the vehicle speed, the vehicle control device firstly calculates a power train control value so that an actual deceleration speed of the vehicle becomes equal to the target deceleration speed, and then outputs the calculated power train control value to the power train control device in order to generate a brake torque by the power train mechanism.
In particular, when the target deceleration speed can be achieved only by using the brake torque generated by the power train mechanism, the vehicle control device repeatedly calculates and outputs the power train control values to the power train control device in order to generate the brake torques by the power train mechanism (hereinafter, referred to as the “brake limitation control”, namely, the “engine braking control”).
On the other hand, when the target deceleration speed cannot be achieved only by using the brake torque generated by the power train mechanism, the vehicle control device outputs the brake control value to the hydraulic brake control device in order to generate an additional brake torque by the hydraulic brake mechanism, in addition to outputs the power train control value to the power train control device in order to generate the brake torque by the power train mechanism (hereinafter, referred to as the “brake use control”).
That is, when the target acceleration calculated by the target acceleration calculation process is a deceleration (as a negative value), and when the magnitude of the brake torque to be generated by the power train mechanism (hereinafter, referred to as an “output request brake torque”) is smaller than the maximum brake torque to be generated by the power train mechanism, the vehicle control device controls only the power train mechanism to generate the brake torque without using the hydraulic brake mechanism.
On the other hand, when the target acceleration calculated by the target acceleration calculation process is a deceleration and when the output request brake torque is greater than the maximum brake torque to be generated by the power train mechanism, the vehicle control device instructs both the power train mechanism and the hydraulic brake mechanism to generate the brake torque, respectively.
In such a type of the vehicle control device, when the vehicle control device switches the brake limitation control to the brake use control in order to generate a brake torque by the hydraulic brake mechanism, a brake control value is set in advance so that the brake torque to be generated by the hydraulic brake mechanism becomes zero [N·m] at the time immediately after the above switching.
A description will now be given of a conventional vehicle control device for performing the above control so that the actual acceleration of the driver's vehicle becomes equal to a target acceleration.
FIG. 11 is a timing chart showing a control operation executed by the conventional vehicle control device.
It will be considered that a target acceleration calculated by the target acceleration calculation step is decreased at timing t1, and finally reaches a negative acceleration corresponding to the maximum brake torque (a deceleration speed) at timing t2, as shown in FIG. 11.
Timing t1′ and timing t2′ shown in FIG. 11 designate a delayed power train control value and a delayed output value which are delayed by the feedback control.
In the above example, the conventional vehicle control device executes the brake limitation control because the output request brake torque is smaller than the maximum brake torque during the period of timing t1 to timing t2. When the output request brake torque is not less than the maximum brake torque at timing t2, the conventional vehicle control device switches the brake limitation control to the brake use control.
Such a conventional vehicle control device has a drawback. That is, when the conventional vehicle control device switches the brake limitation control to the brake use control, it takes a certain period of time until the hydraulic brake mechanism generates an effective brake torque capable of supplying an adequate deceleration to the vehicle, and until the hydraulic brake mechanism generates the brake torque that reaches the target acceleration of the vehicle because the brake torque generated by the hydraulic brake mechanism is zero [N·m] at the timing immediately after the switching is zero [N·m].
That is, the conventional vehicle control device has the problem to cause a time delay until the driver's vehicle completely reaches the target acceleration (deceleration) because a constant deceleration is applied to the driver's vehicle from the state of brake torque of zero [N·m] until the state of generating an effective brake torque, by the hydraulic brake mechanism, to generate the effective acceleration (or deceleration) for the driver's vehicle.
Thus, the conventional vehicle control device often inconveniences the vehicle driver and passengers by the response delay of the actual acceleration of the vehicle from the target acceleration.